The present invention relates generally to the field of optical disc data storage devices and more particularly, but without limitation, to the individual adjustment of pit and land transition locations on an optical disc to improve readback characteristics of the disc.
Optical discs have become increasingly popular as an efficient and cost-effective storage medium for digitally stored data. A typical optical disc comprises a circular disc having, a recording layer of light reflective material embedded in a refractive substrate. The recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data in the form of localized pits and lands (also sometimes referred to as xe2x80x9cmarksxe2x80x9d and xe2x80x9cspacesxe2x80x9d) along a continuously extending spiral track. The length of each pit and land corresponds to one of a selected number of data symbols (for example, from 3T to 11T, with T of determined length).
The data symbols are recovered from the disc through the use of a light source (such as a laser) which applies light of selected wavelength to the rotating disc and a transducer which generates a readback signal indicative of the data in relation to relative differences in reflectivity of the pits and lands. More particularly, it is common to separate the relative elevations of the pits and the lands by a distance equal to a quarter-wavelength of the applied light so as to facilitate a substantial change in the amount of light reflected by the pits as compared to the amount of light reflected by the lands.
One popular optical disc format is commonly referred to as compact disc, or CD, which has found widespread use in recent years in computer applications (such as CD-ROM) and in the music recording industry (audio CDs). A CD has an outer diameter of 120 millimeters (4.724 inches) and a data storage capacity of about 600 megabytes (MB); hence, large computer software applications requiring the use of tens or even hundreds of 3 xc2xd inch, 1.44 MB floppy diskettes can be advantageously installed using a single CD-ROM. Moreover, a typical audio CD can accommodate about 74 minutes and 33 seconds of recorded music and provides improved sound quality through digital recording techniques; accordingly, audio CDs have essentially phased out analog recorded vinyl LP records to become the audio recording medium of choice.
Another popular optical disc format is commonly referred to as digital versatile disc, or DVD. A DVD can be considered a xe2x80x9chigh-densityxe2x80x9d CD, in that a typical DVD has generally the same dimensions as a CD, but can store about 4.7 gigabytes (GB) of data per recording layer, due to increased data storage densities through reductions in pit/land geometries and improvements in data encoding and recovery techniques. Accordingly, DVDs can be advantageously utilized as a storage medium for full-length movies (video DVD), computer storage (DVD-ROM) and music (audio DVD).
As mentioned above, data are stored by an optical disc in relation to selected symbol lengths of the pits and lands. Accordingly, reliable readback of the data requires accurate decoding of the individual lengths of the pits and lands by playback equipment (such as a CD player). Unfortunately, it will be recognized that imperfections typically arise in the mastering/replication process used generate optical discs, so that replicated optical discs (xe2x80x9creplicasxe2x80x9d) can contain small errors that can affect the readback process. Such errors can arise during various steps of the mastering/replication process.
Various efforts have been made in the prior art in an attempt to minimize such errors, such as discussed in U.S. Pat. No. 5,608,711 issued Mar. 4, 1997 to Browne et al., and U.S. Pat. No. 5,486,827 issued Jan. 23, 1996 to Shimizume et al. However, these and other prior art references typically provide global adjustments to the process, necessitating a general trial and error approach in which an attempt is made to bring both shorter and longer pits and lands into acceptable tolerances.
Accordingly, there is a continual need for improvements in the art whereby various errors associated with optical disc mastering/replication processes can be minimized, in order to enhance optical disc quality and manufacturability.
The present invention is directed to an apparatus and method for minimizing errors associated with the writing of data to an optical disc.
In accordance with presently preferred embodiments, an optimization circuit is provided to generate an optimized modulation signal which is used to modulate a writing beam of a laser beam recorder.
The optimized modulation signal comprises a series of data symbols which correspond to resulting pits and lands on the optical disc. The optimization circuit optimizes the locations of pit and land transitions on the optical disc, and hence the lengths of the pits and lands, by individually adjusting leading and trailing edges of the data symbols, as well as amplitudes of the data symbols, in the optimized modulation signal. More particularly, the optimization circuit utilizes delay tables which provide delays of selected duration in relation to nominal lengths of the symbols. In this manner, the particular length of each pit and land on the disc can be individually selected based on nominal symbol length, as well as other factors, including radial and angular locations of the pits and lands on the disc. Moreover, the selective placement of pit and land transitions can be further used to embed a second set of data on the optical disc through the use of pairs of tables having delay values that vary by a selected amount. More particularly, the relative lengths of the pits or lands in a particular area can be adjusted sufficiently to be visibly differentiable (watermarks), while still storing the primary data written to the disc. Likewise, slight differences in the relative lengths of pits and lands over portions of the disc, such as one nanosecond or less, can be advantageously used to provide xe2x80x9chiddenxe2x80x9d data for anti-piracy purposes and the like. In both cases, the second set of data is recorded independent of, and hence does not interfere with the readback of, the primary data.
Finally, electrical jitter errors, including data correlated jitter, can further be minimized through a master driver circuit which can be used to reclock an initial modulation signal prior to generation of the optimized modulation signal by the optimization circuit.
It is therefore an object of the present invention to compensate for errors associated with an optical disc mastering/replication process.
Another object is allow the individual selection of pit and land lengths on an optical disc.
Yet another object is to facilitate the embedding of secondary data on a recording surface independently of the primary data recorded on the surface, to enable the use of watermarks or hidden codes.
Still another object is to eliminate the effects of data correlated jitter, thereby providing audio discs with superior sound quality.
Still another object is to facilitate a significant reduction in cycle times necessary to injection mold replica discs.
These and various other objects, features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.